As is well known, polymeric materials have a macromolecular structure, that is it is characterized by long polymeric chains with variable relative mobility depending on the structure itself, but in all cases such as to allow the access of molecules of substances of lower molecular weight, which insert themselves between the macromolecules forming a real solution.
Obviously the quantity of such substances depends on the molecular nature of the polymer and of the material of low molecular weight. Also, the interaction can be of a chemical and/or physical type.
Chemical interaction, for some chemically more reactive substances, such as acids and/or some organic solvents, can lead to a modification of the polymer itself and in some cases to real dissolution. Conversely, physical interaction is linked to mixing of an essentially reversible type; a solution is created between polymer and low molecular weight substance, with modified physical properties compared to the pure polymer. Since these are substances of low molecular weight, they generally have the effect of facilitating relative movements between the macromolecules, generally producing a lowering of the glass transition temperature (Tg); from a mechanical point of view, the values of the plastic flow σy (tensile) and τy (shear) stress generally decrease, and the elastic modulus E (tensile elastic, or Young's, modulus), and the shear elastic modulus (G), generally decrease. The totality of all these effects is generally termed “plasticization”.
The plasticization depends on the nature of the polymer and on the dissolved low molecular weight substance, and on the quantity of that substance.
The plasticizing substances include organic solvents (for example: MEK, methanol, ethanol, hexane and acetone), and also water. If the polymer is immersed in the plasticizing liquid, it tends to absorb it, and absorbs a certain quantity of it in a time which depends on the rate of diffusion of that substance in the polymer. When an equilibrium situation is reached, that is such as not to have further entry of the plasticizer into the polymer (in reality, at the molecular level, the number of molecules entering is equal to that of those emerging), it is said that the “saturation” content of the plasticizer has been reached, which depends on the chemical nature of the polymer and of the plasticizer and can depend on the temperature.
When the polymer is immersed in a medium having a partial content of plasticizer, the saturation quantity is a function of the percentage of plasticizer present in the medium; more precisely in thermodynamic terms the activity of the plasticizer is referred to. In the case of gaseous mixtures, the activity is linked to the partial pressure; if x is the volumetric fraction of the plasticizer, the partial pressure is equal to x·π, where π is the pressure of the mixture. In the case of water dispersed in air in gaseous form, when there is equilibrium between the gaseous water and the liquid water the activity of the gaseous water is equal to that of the liquid water. In this case, it is stated that the medium is saturated with water and the relative humidity is equal to 100%, and the partial pressure of the water in the gaseous phase is equal to the vapour pressure of the liquid water at the same temperature.
For polymers exposed to media wherein water is present, there is, to a good approximation, a linear relationship between the relative humidity and the percentage quantity of water absorbed by the polymer at saturation.
However, as regards the variability of water absorbed as a function of the temperature, in general the dependence in relation to the temperature is not very great; for the epoxy resins used as matrices for structural composites in the aeronautic industry, the percentage of water absorbed at saturation by the resin immersed in water (or equivalently in humid air at 100% humidity) varies depending on the type of resin. Expressed as Δweight/weight, it typically varies from 1 to 3%, and is almost constant, for the same resin, in the temperature range from 25° to 80° C. [1, 2].
However, the rate at which saturation is reached in the different environments is controlled by the diffusion of the water within the polymer, and is thus a function of the diffusion coefficient, which depends exponentially on the temperature. Integration of the diffusion law leads to the identification of a dependence of the saturation time on the thickness of the quadratic type.
All of the aforesaid leads to the consideration that polymeric materials, such as for example the matrix of composites with a polymeric matrix, are liable in time to absorb atmospheric water to an extent depending on the atmospheric conditions prevailing. In view of the variability of the conditions, it is necessary as a precaution to consider the most unfavourable conditions, which for aeronautic applications have been agreed to be 28° C. and 85% relative humidity for the entire lifetime of the aircraft (typically 30 years). For the majority of composite structures, this involves the hypothesis of assuming a saturation on the scale of at least 85% for certification purposes [3].
As regards the temperatures, generally the minimum temperature (at altitude) is −55° C., and the maximum (on the ground, intense solar exposure) is 80° C.
From what has been said concerning plasticization, the effect of high temperature operates in the same direction as the absorption of moisture; hence the certification of materials and of structures is carried out by assessing the material at high temperature and after absorption of moisture (condition “hot wet”), and at low temperature generally without absorption of moisture (condition “cold dry”).
The need to add these conditions to the aeronautical certification plans, already very onerous because of the mechanical tests at ambient temperature (which in any case concern test pieces, parts, elements, subcomponents and complete components), is very costly in terms of additional experimental activity (even for the exposure of the test samples) and time. Indeed, from the aforesaid, the absorption is very slow, and simulating absorption over thirty years at ambient temperature requires several months even with recourse to accelerated aging (at high temperature).
From the aforesaid, the quantity of moisture which is typically contained in aeronautical composite structures is very variable, and it is even quite rare that it reaches values in equilibrium with an environment with a high percentage humidity. This is because typically the humidity is not so high, and because under the conditions when the aircraft is parked in the sun heating of the irradiated parts occurs which brings them to a temperature greater than ambient, with the accelerated desorption effect due to the heating. Hence, if a direct measurement of the moisture contained in the composite were possible, typically expressed as Δweight/weight, it would be possible to imagine certification of the structures under non-wet conditions (for example at 50% saturation, the condition known as “ambient”), subject to periodical checking on the aircraft composite structure monitored that that percentage is not exceeded. However, at the current state of knowledge, there is no known method for direct measurement of the quantity of humidity. In reality, a very simple method, commonly used in the laboratory, is that of weighing the parts which have absorbed moisture and then to weigh them again after a desorption in a dry environment at high temperature (for example an oven at circa 80° C.), but this method, which gives an average value over the entire thickness of the composite, is obviously not applicable to aeronautical parts in service. Methods based on the measurement of the conductivity or the dielectric properties, or even based on infrared spectrometry, have also been tried, but the results obtained are not satisfactory, above all because the presence of the carbon fibres renders all the properties associated with the resin much more difficult to read; for example, in a composite the thermal or electrical conductivity depends mainly on the fibres, and slight variations in the conductivity of the resin have a very limited effect on the conductivity of the composite. As regards spectrometry, it is difficult to obtain quantitative evaluations of the presence of water from a spectrum obtained on polymerized resin, in particular in the presence of reinforcing fibres.
However, the humidity can be measured with good precision in air, with standardized methods such as that linked to the deformation of a hygrometric substance (for example a hair), or to the comparative reading of the wet bulb and dry bulb temperature or (more recently) with a capacitive humidity sensor, that is a condenser which changes its capacity as a function of the humidity of the air between the opposed conductors (or plates).